Western and Gulf Arab countries that back Assad's opponents have called plans for an election a "parody of democracy" and said it would wreck efforts to negotiate a peace settlement.
United Nations-backed talks in Geneva collapsed in February with both sides far from agreement - not least over the question of whether Assad should go.
Monzer Akbik of the Western-backed National Coalition opposition group, told Reuters the election was a sign Assad was unwilling to seek a political solution to the conflict.
"This is a state of separation from reality, a state of denial. He didn't have any legitimacy before this theatrical election and he will not after," Akbik said.
"We do not know what actor he is putting up as an opponent but we are not taking this seriously."
Infighting has fragmented the anti-Assad forces, and several major opposition figures did not attend the Geneva talks.
The European Union reiterated its stance against holding an election now, saying such a vote: "conducted in the midst of conflict, only in regime-controlled areas and with millions of Syrians displaced from their homes would be a parody of democracy, have no credibility whatsoever, and undermine efforts to reach a political solution."
The three-year-old rebellion against Assad has killed more than 150,000 people, forced millions to flee their homes and caused the government to lose control over swathes of territory.
On Monday, 11 government and loyalist fighters were killed near Talbisah, an Alawite town north of the central city of Homs, the Syrian Observatory for Human Rights.
Alawites are followers of an offshoot of Shi'ite Islam. Assad is Alawite and the bulk of his opponents are Sunni Muslims, who form the majority of Syria's population.